Calibrated load cell

ABSTRACT

A calibrated load cell includes a monolithic load beam having a first region, a second region on a distal end of the load beam from the first region for receiving a force from a load, and a third region arranged between the first and second regions, wherein the third region comprises a recess on one side of the load beam. A strain gauge is arranged in the recess for detecting a deformation of the third region from the load and for generating a strain gauge output signal proportional to the deformation of the third region. The load cell also includes a microcontroller arranged in the recess for receiving and processing the strain gauge output signal to produce a load cell output signal that represents the load on a load cell output cable. The microcontroller transforms the strain gauge output signal based on calibration parameters to produce the load cell output signal as a calibrated load cell output signal.

FIELD OF THE INVENTION

This invention relates to load cells and methods for calibrating loadcells and, more particularly, to a method for calibrating a load cell atthe point of manufacture.

BACKGROUND OF THE INVENTION

Knowledge of the weight of a grain bin and its contents is a commonmethod for assessing the amount of grain that may be stored in the bin.This may be accomplished by instrumenting each of the support legs ofthe grain bin with a load cell and determining the total weight byaggregating the load experienced at each support leg. One of theinherent challenges to accurately determine the total weight of thegrain bin in such a system is the need for calibrated load cells. As isknown, the cost of calibrated load cells is substantially higher thanuncalibrated load cells. In some applications, the cost of a calibratedstrain gauge (i.e., one in which the output signal is linearly varyingwith weight with known slope and offset) is quite high. For larger grainbins, e.g., those supported by eight legs, it is not uncommon for thecost of calibrated strain gauges to exceed the cost of the grain binitself.

The prior art describes various forms of calibrated load cells. Thesecalibrated load cells are adjusted individually at the factory toprovide the load cells with uniform offset and sensitivity factors.However, calibration and subsequent testing of load cells at the factorycan raise the cost for manufacturing products that incorporate the loadcells. To reduce this cost, previous devices calibrate load cells onproducts either after those products leave the factory or after theuncalibrated load cells have been installed in the field. For example,many calibrated load cells available in the market have calibrationparameters printed on paper that must be manually entered into themeasurement system or instrumentation that reads the load cell values.The instrumentation that reads the load cell then performs thecalibrating transformation, changing the reading that is reported by theload cell into a calibrated reading.

U.S. Pat. No. 6,636,820 to Livingston addresses this issue by enablingthe use of uncalibrated load cells to instrument a grain bin anddescribes a procedure for performing a calibration in the field. Thisapproach requires that the grain bin be loaded with different knownamounts of grain to perform the calibration. It also assumes that eachuncalibrated load cell responds linearly to weight applied to the leg ofthe grain bin. U.S. Pat. No. 4,738,324 to Borchard describes methods todetermine load cell offsets, scale factors, and the effects ofstructural flexure and distortion in the factory, and then programsunique calibration factors into each load cell. To accomplish this typeof calibration, however, the load cells must be installed prior toshipment from the manufacturer. This is impractical where the load cellsmust be field-installed onto a storage device such as a grain bin.Similarly, U.S. Pat. No. 6,112,162 to Richards, attempts to solve thisproblem by performing an on-site calibration using multiple weights.Each weight is placed on top of each individual load cell, effectivelydetermining the scale factor for that individual load cell. This processis repeated for each of the individual load cells, resulting in a timeconsuming operation.

The present invention addresses these issues in enabling acost-effective calibrated load cell to be provided to the agriculturalmarket and provides for calibration of an uncalibrated load cell and thesafe retention of calibration parameters at the time of manufacture. Thepresent invention removes the requirement for manually enteringcalibration parameters or performing calibration in the field. It alsoaddresses the potential for non-linearities in the responsecharacteristic of the load cell, thus providing a linear reading evenunder conditions in which the strain gauge signal is not linear. Inaddition, the invention described herein provides security from unwantedtampering with the system.

SUMMARY OF THE INVENTION

According to one aspect of the present invention, a calibrated load cellincludes a monolithic load beam having a first region, a second regionon a distal end of the load beam from the first region for receiving aforce from a load, and a third region arranged between the first andsecond regions, wherein the third region comprises a recess on one sideof the load beam. A strain gauge is preferably arranged in the recessfor detecting a deformation of the third region from the load and forgenerating a strain gauge output signal proportional to the deformationof the third region. The load cell also includes a microcontrollerarranged in the recess for receiving and processing the strain gaugeoutput signal to produce a load cell output signal that represents theload on a load cell output cable. The recess having the strain gauge andmicrocontroller therein may be encapsulated in a potting material.

Preferably, the microcontroller transforms the strain gauge outputsignal based on calibration parameters to produce the load cell outputsignal as a calibrated load cell output signal. The calibrationparameters may be stored in a memory accessible to the microcontrollerafter the recess has been encapsulated in the potting material. Thecalibration parameters may comprise a plurality of ordered pairs eachcomprising an ADC count and an associated output current and themicrocontroller may compute the load cell output signal by assuming alinear response between each of the ordered pairs.

The microcontroller may be coupled to a calibration fixture thatcommunicates the calibration parameters to the microcontroller and mayprovide a signal to the calibration fixture indicating that themicrocontroller is capable of receiving the calibration parameters fromthe calibration fixture, which may be for only a short period of timefollowing the signal from the microcontroller to the calibrationfixture.

According to another aspect of the invention, a method for providingcalibration parameters to the calibration load cell includes connectingthe load cell output cable to the calibration fixture; imposing a weightselected from a plurality of known weights on the second region of theload beam; communicating by the calibration fixture a desired load celloutput signal corresponding to the known weight to the microcontroller;receiving by the microcontroller the strain gauge output signal from thestrain gauge at the known weight; storing in the memory the strain gaugeoutput signal and the desired load cell output signal as one of theplurality of ordered pairs in the memory; modifying the weight imposedon the second region of the load beam; and repeating these steps foreach of the plurality of known weights.

Yet another aspect of the invention includes a method for providingcalibration parameters to the calibrated load cell wherein the load cellhas an initial set of calibration parameters, the method includingconnecting the load cell output cable to the calibration fixture;communicating the initial set of calibration parameters from thecalibrated load cell to the calibration fixture; repeatedly imposing aweight selected from a plurality of known weights on the second regionof the load beam; receiving by the calibration fixture the load celloutput signal; computing by the calibration fixture an error valuebetween a weight reported via the load cell output signal and the knownweight; storing the error value associated with the known weight by thecalibration fixture; determining the maximum error value from theplurality of error values; adjusting at least one of the calibrationparameters based on the error value associated with one of the pluralityof known weights and storing a revised set of calibration parameters inthe calibration fixture; and repeating these steps until the maximumerror value falls below a desired threshold.

In another embodiment, any error attributable to the digital to analogconverter in the microcontroller to generate the load cell output signalmay be accounted for with a DAC calibration process in which a set ofDAC calibration parameters is communicated from the calibration fixtureto the calibrated load cell. The DAC calibration parameters may includetwo ordered pairs each comprising an ADC count and a DAC countcorresponding to an expected output current, wherein the first orderedpair representing a large weight imposed on the load beam and the secondordered pair representing a small weight imposed on the load beam. Thecalibration fixture may communicate a first DAC calibration signal tothe load cell such that the load cell output signal is based on thefirst ordered pair and a second DAC calibration signal such that theload cell output signal is based on the second ordered pair. Thecalibration fixture may then process the load cell output signals todetermine a DAC correction transform that accounts for any error withinthe load cell in producing the load cell output signal from the DACcount and this transformation may be applied to the calibrationparameters stored in the memory.

A further embodiment of the invention includes a method forbi-directional digital communication to and from a device having asensor for accumulating information, a communication cable coupling thedevice to an external system, a microcontroller for processing andcommunicating the processed information to the external system via thecommunication cable as a current signal the device by (i) communicatingdigital information from the device to the external system by settingthe output current less than an output low threshold for a logical 0 andhigher than an output high threshold for a logical 1; and (ii)communicating digital information from the external system to the deviceby the external device establishing a voltage less than an input lowvoltage for a logical 0 and higher than an input high voltage for alogical 1.

BRIEF DESCRIPTION OF THE FIGURES

These and other features, aspects and advantages of the invention willbecome more fully apparent from the following detailed description,appended claims, and accompanying drawings, wherein the drawingsillustrate features in accordance with an exemplary embodiment of thepresent invention, and wherein:

FIG. 1 is an illustration of a grain bin having a load cell installed atthe base of each leg;

FIG. 2 is a top perspective view of an exemplary load cell;

FIG. 3 is a bottom perspective view of an exemplary load cell;

FIG. 4 is another bottom perspective view of an exemplary load cell;

FIG. 5 is a further bottom perspective view of an exemplary load cell;

FIG. 6 is a graph mapping the ADC counts to the output current of theload cell;

FIG. 7 is another graph mapping the ADC counts to the output current ofthe load cell;

FIG. 8 is a further graph mapping the ADC counts to the output currentof the load cell;

FIG. 9 is a circuit diagram of a load cell interface circuit;

FIG. 10 is a diagram of a circuit for reading the output current of theload cell interface circuit; and

FIG. 11 is a circuit diagram illustrating one embodiment of an interfacecircuit for calibration of a load cell.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates a conventional bin 10 having a plurality of legs 12used for storing granular material such as grain. In one embodiment, oneor more of the legs 12 of the bin 10 may be instrumented with a loadcell 14 designed to aid in measuring the weight and, therefore, thequantity, of granular material stored in the bin 10. Various embodimentsmay be described in connection with a calibrated load cell for use witha grain bin, although the systems and methods described herein may beused with other forms of systems for measuring granular material.

FIGS. 2-5 illustrate one embodiment of a load cell 14. The load cell 14may be in the form of a monolithic load beam having a square orrectangular cross section, although other geometries and types of loadcells are within the scope of this invention, including any suitablerigid structure capable of deforming under a load or strain. The loadcell 14 may be characterized as having three regions along itslongitudinal extension. The first region 16 may be affixed to astationary object using, for example, bolts passing through one or morechannels 22 extending through the first region 16. The second region 18encompasses the force application side of the cell 14 and may alsoinclude one or more channels 22 for receiving bolts or other means forattaching the load cell 14 to the object to be measured, for example,the bin 10. The third region 20 is the deformation section lying betweenthe first region 16 and the second region 18 and preferably includescomponents for measuring the strain placed on the second region 18 ofthe load cell 14 as described below. The top side of the load cell 14,as shown in FIG. 2, may include a three-conductor cable 24 forcommunicatively connecting the load cell 14 for power, signaltransmission, and calibration, as further described below.

FIG. 3 illustrates the bottom side of the load cell 14, which includes apocket or recess 30 for housing the electronics built into the load cell14. The recess 30 includes sensors to detect the deformation of the loadcell under applied weight such as strain gauge sensors 32 and, as shownin FIG. 4, a ribbon cable 34 coupling the sensors 32 to a printedcircuit board 36. The strain gauge sensors 32 may be arranged to form aWheatstone bridge and may be of a variety of types including piezoresistor strain gauges, fiber optic gauges, capacitive strain gauges, orany other suitable gauge. The circuit board 36 includes a plurality ofelectronics, including an analog-to-digital converter, to provide signalprocessing of the output signals from the sensors 32, which arepreferably in the form of ADC counts. The board 36 also includes a formof microcontroller and non-volatile memory. To provide environmentalprotection and security from electronic tampering, the recess 30,containing the sensors 32, the cable 34, and the board 36, may be pottedwith a potting material 38 as shown in FIG. 5. Preferably, the pottingmaterial 38 is flush with or below the surface of the load cell 14.

In use, the weight of an individual leg of the bin 10 is applied to thesecond region 18 of the load cell 14 via a mechanical assembly,generating a deformation in the deformation area, i.e., third region 20,by the shear movement of the force applied to the second region 18. Thisdeformation may be detected via the strain gauge sensors 32, which thenemit a measurement signal corresponding to the deflection to the board36 via cable 34. The circuit board 36 contains electronics thattranslate the strain information from the strain gauge sensors 32 into acalibrated output current signal on cable 24 that represents the load onthe leg of the bin 10 in which the load cell 14 is installed.

Post manufacture, but prior to shipment to the field, the finished loadcell 14 of FIG. 5 may be placed in a fixture for calibration asdescribed below. The calibration parameters are stored in thenon-volatile memory of the circuit board 36 so that they are retainedwhen power is not provided to the load cell 14. During operation, thecalibrating transformation from strain gauge sensors 32 to load cellcurrent output is performed within the microcontroller on the board 36of the load cell 14 itself.

The microcontroller that is installed within the load cell 14 performsthe transduction or transformation operation, translating ADC countsfrom an instrumentation amplifier monitoring the output of the straingauge sensors 32 on the ribbon cable 34 into an output current signalrepresenting the load on the cell 14. In one embodiment, the ADC countsmay range from approximately 2500 at bottom of the scale to 9000 at topof the scale (using a 14-bit A/D conversion) and are transformed into anoutput current signal of 400 μA at bottom of the scale (empty) to 1400μA at top of the scale (full).

If this transformation is linear, the above two points can be used toestablish the expression i=mc+b, where i is the output current signal(in mA), c is the ADC counts from the strain gauge sensors 32, and m isthe slope, which may be expressed as

$m = {( \frac{{1400\mu A} - {400\mu A}}{9000 - 2500} ) = {0.154^{\mu A}/{ADCcount}}}$

and b is the offset, b=400 μA−0.154(2500)=15 μA or i=0.154c+15 μA. Thisembodiment is illustrated in the graph of FIG. 6, and corresponds to thecase where two calibration points are entered.

As is known, the response from the strain gauge sensors 32 may not belinear over a range of weight values. For example, the true response ofthe strain gauge sensors 32 may dip below the straight line responseillustrated in FIG. 6. This is illustrated using a 4-point piece-wiselinear transformation calibration in FIG. 7, where the calibrationvalues are as follows:

ADC Counts Current (μA) 2500 400 5000 720 7000 1010 9000 1400

In FIG. 7, the dashed line represents the linear transformationcorresponding to only the first and last calibration points. Thepiece-wise linear transformation that follows all of the calibrationpoints is clearly below the dashed line.

As another example, the true response of the strain gauge sensor mightrise above the straight line of FIG. 6. This is illustrated using a5-point piece-wise transformation calibration in shown in FIG. 8, wherethe calibration values are as follows:

ADC Counts Current (μA) 2500 400 3000 550 6000 1010 7500 1200 9000 1400

Again, in FIG. 8, the dashed line represents the linear transformationcorresponding to only the first and last calibration points. Thepiece-wise linear transformation that follows all of the calibrationpoints is clearly above the dashed line in this case.

Using the current signaling wire (as described above), the actualresponse curve can be mimicked to whatever degree is desired byproviding any number of calibration points (in the preferred embodiment,the limit is set to 8 points). The response of the load cell 14 is thendetermined to be the piece-wise linear concatenation of lines betweenthese calibration points. An additional constraint is that the pointsmust be continuously increasing (i.e., the individual line segments musthave positive slope, and the overall response curve is monotonicallyincreasing).

The electronics on the circuit board 36 include a microcontroller thatperforms the piece-wise linear transduction described in the previoussection. During the manufacturing process, embedded software executingon the microcontroller is loaded into non-volatile memory associatedwith the microcontroller via a connector on the circuit board 36.

As described above, preferably at a later stage in the manufacturingprocess, after the circuit board 36 has been mounted into the recess 30in the bottom of the load cell 14 (see FIGS. 3 and 4), the circuit board36 is covered with potting material 38. This makes it impractical toreprogram the microcontroller, enhancing the security of the system.Even physical contact with the load cell 14 does not give a maliciousactor access to a programming interface (connector) on themicrocontroller.

Nominally, this physical protection associated with the programminginterface would also preclude the ability to store calibrationparameters in the non-volatile memory associated with themicrocontroller. However, it is advantageous to calibrate the load cell14 late in the manufacturing process. Specifically, the calibrationshould occur after the potting material 38 has been installed and cured,since the physical properties of the potting material can potentiallyimpact the transfer of the weight on the load cell 14 to the signal fromthe strain gauge sensor 32.

A mechanism to communicate calibration parameters to the microcontrollerwithout the use of the regular programming interface (which isinaccessible due to the potting material 38), enables this by supportinga combination analog/digital communication via the 3-conductor cable 24used for normal operation.

Preferably, the three conductors on the cable 24 include a groundreference, power (e.g., +5 V power to the load cell 14), and a signalconductor indicating the weight applied to the load cell 14. To enablethe communication of the calibration parameters to the microcontrollerinstalled on the circuit board 36 after the potting material 38 has beeninstalled and cured, the purpose of the signal conductor 24 may beexpanded to support bidirectional communication.

For signaling from the load cell 14, the communication across theconductor 24 is a current signal. Under normal operation, this currentsignal indicates weight. However, under software control of themicrocontroller, it is possible to send digital information via thiscurrent signal. For example, any current less than a specified threshold(nominally one-half scale, or 900 μA) is received as a LOW digit and anycurrent greater than the threshold is received as a HIGH digit.

In addition, individual fixed current levels may be defined to havespecific meaning. For example, a fixed output of the top of the scalemight indicate “invalid calibration parameters.” In the preferredembodiment, the load cell 14 outputs a current of 150 μA to indicate“waiting for calibration data.”

For signaling to the load cell 14, the communication may be a voltagesignal. FIG. 9 illustrates a preferred load cell interface circuit 40.In this circuit, the desired output current is established by themicrocontroller 42 at V_(out), with the output current I_(out)effectively V_(out)/1 kΩ. This output current may be read at node 48 bya calibration fixture, for example, using the circuit 50 illustrated inFIG. 10. This node 48 is identified as I_(out)/CAL_(in) to represent itsdual purpose acting as both a node for transmitting the output currentof the load cell 24 in normal operation and as operating as inputcalibration voltage during the calibration process.

When the I_(out)/CAL_(in) pin 48 of the load cell 14 is connected to thecommunication interface of the calibration fixture 60, the result is thecircuit of FIG. 11. In this circuit, the “Signal wire” connects theI_(out)/CAL_(in) node 48 of the load cell 14 with the communicationinterface of the calibration fixture 60.

With the switch 54 in the calibration fixture connected to Vdd, thecurrent signal i from the load cell 14 establishes a voltage Vin=i⋅1 kΩacross the precision resistor 52 in the fixture 60, enabling the fixture60 to read the current signal from the load cell 14.

The present invention also provides a voltage signal communicated fromthe calibration fixture 60 to the load cell 14. First, to send a logicalzero (low level) to the load cell 14, the calibration fixture 60 setsthe switch 54 to GND (0 V). Although illustrated as a switch, in apreferred embodiment, the switch 54 may be constructed usingtransistors. Under this circumstance, independent of the value of Voutin the load cell 14, the voltage at D_(in) within the load cell 14 is alogical low (below the “low” threshold of the digital input D_(in)).D_(in) is connected to a digital input pin of the microcontroller.

To send a logical one (high level) to the load cell 14, the calibrationfixture 60 sets the switch 54 to V_(dd). In this case, the voltage atD_(in) will depend upon the output current i. At top of scale (i=1400μA), D_(in) is approximately Vdd−i⋅1.2 kΩ=3.32 V (if Vdd=5V), which iswell above the “high” threshold at D_(in). At bottom of scale (i=400μA), D_(in) is approximately V_(dd)=i⋅1.2 kΩ−4.52 V (if V_(dd)=5V),which is also well above the “high” threshold at D_(in).

In this way, the calibration fixture 60 may communicate digitalinformation to the load cell 14. If the switch is controlled by the TXline of an asynchronous UART on the calibration fixture 60, and theD_(in) is connected to the RX line of an asynchronous UART on the loadcell 14, serial communication is straightforward. In the embodimentdescribed, the data rate is fairly low (at 200 Kbits/s); however, anydata rate that can be sustained by the loading effects of the cable 24can be supported.

Preferably, an additional feature to reduce the chances of an accidental(or malicious) recalibration may be incorporated based on the timingrequirements of recalibration. On power-up, the load cell 14 may send ananalog output signal of 150 μA to indicate it is “waiting forcalibration data.” The load cell 14 maintains this output for a shortamount of time, for example, 5 seconds. During this time window, theload cell 14 is receptive to a calibration message. After this timeperiod elapses, the output signal reverts to signaling the weightincident on the load cell 14, and no calibration messages areacknowledged.

In embodiments where one-way communication from the calibration fixtureto the load cell 14 is sufficient, maintaining the output current at alow value has the additional benefit that it provides maximum noisemargins for the “high” voltage at D_(in). In this case, (i=150 μA), andD_(in) is approximately Vdd−i⋅1.2 kΩ=4.82 V (if Vdd=5V).

There are multiple methods by which one can determine the calibrationpoints illustrated in FIGS. 6, 7, and 8. As illustrated in FIGS. 6, 7,and 8, the calibration points may be represented by a set of orderedpairs, (count_(i), current_(i)), where i ranges from one to the numberof calibration points.

One approach to determining these points is for the microcontrollersoftware to provide information to the calibration fixture 60 as to theactual value of the ADC counts at a discrete set of imposed weights. Inthis embodiment, the calibration fixture 60 imposes a known weight onthe load cell 14, the load cell 14 provides the ADC counts at thatweight to the calibration fixture 60, and this process is repeated forseveral different weights. The communication of the ADC counts can occurin at least two different ways: (1) using the digital communicationtechniques described above, or (2) adopting the linear transductioncurve of FIG. 6 and communicating the ADC counts as an analog signal(which is then read at the Vin+ and Vin− terminals on the calibrationfixture 60).

A second approach is for the calibration fixture 60 to impose a set ofweights on the load cell 14 and, for each distinct value of weight, thecalibration fixture 60 communicates the desired output current to theload cell 14, the load cell 14 samples the ADC counts, and then the loadcell 14 stores the (count_(i), current_(i)) pair as one of itscalibration points.

Each of the above approaches has merit, but they both make thesimplifying assumption that the digital-to-analog converter for themicrocontroller on the load cell 14 and the voltage-to-current circuitryillustrated in FIG. 9 both give a linear response. A third preferredapproach does not make this assumption, and will correct fornon-linearities throughout the load cell 14 signal path (from mechanicalstress all the way to output current signal). The iterative approach canbe expressed in terms of the following greedy algorithm: starting withan initial set of calibration parameters (e.g., those of FIG. 6), loopover a set of fixed, known weights applied by the calibration fixture60, measuring the current output at each position. The error between theknown weight and the desired output for each point is then computed bythe calibration fixture 60. For one of the points that has a non-zeroerror (e.g., the point with the maximum error), the calibrationparameter is adjusted at that point to diminish the error at that point.Looping over the set of fixed, known weights continues until the maximumerror is below a given desired amount.

Although, this iterative approach described above is in the form of agreedy algorithm, any iterative meta-heuristic approach may be used,such as, for example, simulated annealing as illustrated in S.Kirkpatrick, C. D. Gelatt Jr., M. P. Vecchi, Science, 13 May 1983: Vol.220, Issue 4598, pp. 671-680. DOI: 10.1126/science. 220.4598.671;genetic algorithms as illustrated in M. Mitchell, An Introduction toGenetic Algorithms. MIT Press, 1998; threshold acceptance as illustratedin G. Dueck and T. Scheuer, Journal of Computational Physics, 1990: Vol.90, Number 1, pp. 161- 175. DOI: 10.1016/0021-9991(90) 90201-B, as wellas other algorithms known to those skilled in the art.

Using the techniques above, calibration parameters support both linearand non-linear transduction. In the preferred embodiment, thistransduction takes place within the load cell 14 itself. In analternative embodiment, the calibration parameters are stored in theload cell 14, but the transduction may be performed externally to theload cell 14, for example, in the instrumentation to which the load cell14 is attached. In this alternative embodiment, the bi-directionalcommunications capability described above may be used to communicate thestored calibration parameters to the external instrumentation.

Those skilled in the art will appreciate that any imprecision in theresponse of the load cell, i.e., translating the response from thestrain gauge into an output current, has two components. The first isany error or imprecision in converting the strain gauge 32 reading toADC counts within the microcontroller 42. The second is any error inconverting the DAC counts within the microcontroller 42 to an outputcurrent.

As is known, during the calibration procedure, calibration parametersare introduced in a digital form and are converted by the load cell 14via an internal digital-to-analog converter (DAC). Yet anothercalibration procedure preferably includes adding a discrete DACcalibration step to the process in order to characterize any errorintroduced by the DAC in the load cell 14 during the calibration processand, therefore, account for any error caused by the DAC. To accomplishthis, sets of fixed-output calibration data are generated by calibrationfixture 60 and are loaded into the load cell 14 as described above.Preferably, this data is generated in a manner in which the load cell 14views it as valid calibration data (e.g., each calibration point must begreater than the previous point). However, this fixed-output calibrationdata is exaggerated such that the load cell 14 effectively ignores theinternal ADC reading and outputs a fixed current.

For example, the calibration data may include a pair of ADC countsarranged such as (Actual, Expected), where “Actual” represents thedigital value read by the ADC within the load cell 14 (i.e., ADC counts)and “Expected” represents the digital value representing the outputcurrent that should have been read by the ADC assuming no error due tothe DAC. Thus, if the load cell 14 had a perfect response that requiredno calibration, the values for “Actual” and “Expected” would beidentical. During this process, the calibration data must include atleast two pairs of ADC counts and linear interpolation may be usedbetween each adjacent pair.

As an example, assume that the load cell 14 reports a current of 400 μAfor an ADC count of 2500 (representing no load on the load cell 14) anda current of 1400 μA for an ADC count of 9000 (representing full load).The calibration fixture 60 may force the ADC count to a digital value ofapproximately 2500 (corresponding to 400 μA) regardless of what theactual ADC count is by sending calibration data of (2500, 2500) (9000,2501) to the load cell 14. Thus, if the actual ADC count is 2500, theADC reports 2500, and if the actual ADC count is 9000, the ADC reports2501. Thus, the entire range of expected inputs would be compressed toan output range of only two digital values (2500-2501). In a similarfashion, in order to force the ADC count to a digital value ofapproximately 3800 (corresponding to 600 μA) regardless of what theactual ADC count is, the calibration fixture 60 may send calibrationdata of (2500, 3800) (9000, 3801).

Thus, for this process, the calibration fixture 60 sends a sequence ofdifferent calibration data sets to the load cell 14 for differentexpected output values and records the current read by the calibrationfixture 60 for that value along with the digital value that wasrequested. Using this information, any error due to the DAC may becharacterized for the load cell 14. A transform (again, linearlyinterpolated) may be created to provide the digital value that thecalibration fixture 60 must request to receive any desired currentoutput. The new 1:1 calibration data may then loaded back into the loadcell 14 and the calibration procedure continues as described above. ThisDAC correction transform is applied to the calibration points generatedand sent to the load cell 14 for the remainder of the calibrationprocess. Thus, any error attributed to the DAC has been accounted for,and there should be no calibration error on the input side.

Although the ability to incorporate bi-directional communicationcapability has been illustrated using the calibrated load cell describedherein, those skilled in the art will recognize that such bi-directionalcommunication may be incorporated into any device that normallytransmits status information or data one way. For example, a devicehaving some form of sensor for measuring or otherwise accumulatinginformation, and a form of microcontroller for communicating themeasured or accumulated information (possibly after processing by themicrocontroller) to an external system via a nominally one-waycommunication means may be improved by permitting bi-directionalcommunication to and from such device over the communication means.Thus, if the device is designed to output its results as a currentsignal over a communication cable that is coupled to the externalsystem, the external system may communicate information to the device byplacing a voltage signal on the communication means that may be actedupon by the microcontroller.

For example, bi-directional digital communication between the device andthe external system may include communicating digital information fromthe device to the external system by setting, by a microcontrollerwithin the device, an output current less than an output low thresholdfor a logical 0 and higher than an output high threshold for alogical 1. In a like fashion, communicating digital information from theexternal system to the device may be accomplished by the external deviceestablishing a voltage less than an input low voltage threshold for alogical 0 and higher than an input high voltage threshold for alogical 1. Each of the device and the external system may include aUART, and the bi-directional communication that occurs between these twosystems is enabled via the UARTs. Communication between the device andthe external system may be initiated by either—for example,communication from the external system to the device may be initiated bythe external system communicating an input communication signal having avoltage level substantially lower than the input low voltage thresholdto the device, which may indicate that the external system is ableand/or willing to communicate with the device. Similarly, communicationfrom the device to the external system may be initiated by the devicecommunicating an output communication signal having an output currentsubstantially greater than the input high threshold to the externalsystem, which may indicate that the device is able and/or willing tocommunicate with the external system.

The present invention improves the prior art in a number of ways. Forexample, to support non-linearity in the response of the strain gaugesensors 32, the calibration process utilizes a fixed, but arbitrarynumber of calibration points, and the output current signal of the loadcell 14 is computed as the piece-wise linear concatenation of thesecalibration points. In addition, as a security feature, the programstored in the embedded microcontroller on the circuit board 36 isinaccessible after the potting material has cured. However, it isdesirable to perform the calibration of the load cell 14 after thepotting process, as the mechanical properties of the potting materialcan impact the calibration of the load cell 14. To address this, thecalibration of the load cell 14 may be performed by bi-directionallycommunicating with the microcontroller using the same signal wire 24that is used for the output current signal from the load cell 14 innormal operation. Preferably, this communication to the load cell 14 isconstrained to a limited time period after power-up of the load cell 14.

Although certain illustrative embodiments and methods have beendisclosed herein, it will be apparent from the foregoing disclosure tothose skilled in the art that variations and modifications of suchembodiments and methods may be made without departing from the spiritand scope of the invention. For example, those skilled in the art willrecognize that the present invention may be readily adapted to fit anyconfiguration where field calibration of uncalibrated load cells isrequired. In addition, although many of the embodiments illustratedherein are used in connection with determining the weight of, or amountof grain in, a bin, the systems and methods described herein may be usedin connection with load cells used in many applications. Accordingly,the invention should be limited only to extent required by the appendedclaims and the rules and principals of applicable law.

1. A calibrated load cell comprising: a monolithic load beam having afirst region, a second region on a distal end of the load beam from thefirst region for receiving a force from a load, and a third regionarranged between the first and second regions, wherein the third regioncomprises a recess on one side of the load beam; a strain gauge arrangedin the recess for detecting a deformation of the third region from theload and for generating a strain gauge output signal proportional to thedeformation of the third region; a load cell output cable forcommunicating a load cell output signal that represents the load; and amicrocontroller arranged in the recess for receiving and processing thestrain gauge output signal to produce the load cell output signal;wherein the recess having the strain gauge and microcontroller thereinis encapsulated in a potting material.
 2. The calibrated load cell ofclaim 1 wherein the load cell output signal is a current signal.
 3. Thecalibrated load cell of claim 1 wherein the microcontroller transformsthe strain gauge output signal based on calibration parameters toproduce the load cell output signal as a calibrated load cell outputsignal.
 4. The calibrated load cell of claim 3 wherein the calibrationparameters are stored in a memory accessible to the microcontroller. 5.The calibrated load cell of claim 3 wherein the calibration parametersare stored in the memory after the recess has been encapsulated in thepotting material.
 6. The calibrated load cell of claim 3 wherein thecalibration parameters are transferred to the load cell via the loadcell output cable.
 7. The calibration load cell of claim 3 wherein thecalibration parameters comprise a plurality of ordered pairs eachcomprising an ADC count and an associated output current.
 8. Thecalibrated load cell of claim 7 wherein the microcontroller computes theload cell output signal by assuming a linear response between each ofthe ordered pairs.
 9. The calibrated load cell of claim 8 wherein theload cell output cable is communicatively connected to a calibrationfixture.
 10. The calibrated load cell of claim 9 wherein themicrocontroller communicates status information to the calibrationfixture via the load cell output cable.
 11. The calibrated load cell ofclaim 9 wherein the calibration fixture communicates the calibrationparameters to the microcontroller.
 12. The calibrated load cell of claim11 wherein the calibration parameters are communicated to themicrocontroller as voltage signals over the load cell output cable. 13.The calibration load cell of claim 11 wherein the microcontrollerprovides a signal to the calibration fixture indicating that themicrocontroller is capable of receiving the calibration parameters fromthe calibration fixture.
 14. The calibration load cell of claim 11wherein the microcontroller is capable of receiving the calibrationparameters for only a short period of time following the signal from themicrocontroller to the calibration fixture.
 15. A method for providingcalibration parameters to the calibration load cell of claim 9comprising the steps of: a. connecting the load cell output cable to thecalibration fixture; b. imposing a weight selected from a plurality ofknown weights on the second region of the load beam; c. communicating bythe calibration fixture a desired load cell output signal correspondingto the known weight to the microcontroller; d. receiving by themicrocontroller the strain gauge output signal from the strain gauge atthe known weight; e. storing in the memory the strain gauge outputsignal and the desired load cell output signal as one of the pluralityof ordered pairs in the memory; f. modifying the weight imposed on thesecond region of the load beam; and g. repeating steps b through f foreach of the plurality of known weights.
 16. A method for providingcalibration parameters to the calibrated load cell of claim 9, whereinthe memory accessible to the microcontroller comprises an initial set ofcalibration parameters, the method comprising the steps of: a.connecting the load cell output cable to the calibration fixture; b.communicating the initial set of calibration parameters from thecalibrated load cell to the calibration fixture; c. imposing a weightselected from a plurality of known weights on the second region of theload beam; d. receiving by the calibration fixture the load cell outputsignal; e. computing by the calibration fixture an error value between aweight reported via the load cell output signal and the known weight; f.storing the error value associated with the known weight by thecalibration fixture; g. repeating steps c through f for each of theplurality of known weights; h. determining the maximum error value fromthe plurality of error values; i. adjusting at least one of thecalibration parameters based on the error value associated with one ofthe plurality of known weights and storing a revised set of calibrationparameters in the calibration fixture; and j. repeating steps c throughi until the maximum error value falls below a desired threshold.
 17. Themethod of claim 16 further comprising the steps of communicating therevised set of calibration parameters to the calibrated load cell. 18.The method of claim 16 further comprising the steps of: communicating aset of DAC calibration parameters from the calibration fixture to thecalibrated load cell, wherein the set of DAC calibration parameterscomprises two ordered pairs each comprising an ADC count and a DAC countcorresponding to an expected output current for such ADC count, thefirst ordered pair representing a full load imposed on the load beam andthe second ordered pair representing no load imposed on the load beam;communicating a first DAC calibration signal from the calibrationfixture to the load cell such that the load cell output signal is basedon the first ordered pair; receiving by the calibration fixture the loadcell output signal; communicating a second DAC calibration signal fromthe calibration fixture to the load cell such that the load cell outputsignal is based on the second ordered pair; receiving by the calibrationfixture the load cell output signal; processing the load cell outputsignals to determine a DAC correction transform that accounts for anyerror within the microcontroller in producing the load cell outputsignal from the DAC count; and applying the DAC correction transform tothe calibration parameters stored in the memory.
 19. In a system havinga device comprising a sensor for accumulating information, acommunication cable coupling the device to an external system, amicrocontroller for processing and communicating the processedinformation to the external system via the communication cable as acurrent signal, a method for bi-directional digital communicationbetween the device and the external system comprising the steps of:communicating digital information from the device to the external systemby setting, by the microcontroller, the output current less than anoutput low threshold for a logical 0 and higher than an output highthreshold for a logical 1; and communicating digital information fromthe external system to the device by the external device establishing avoltage less than an input low voltage threshold for a logical 0 andhigher than an input high voltage threshold for a logical
 1. 20. Themethod of claim 19 wherein the device is a load cell.
 21. The method ofclaim 19 wherein the device and the external system each comprise a UARTand the bi-directional communication occurs between the UARTs.
 22. Themethod of claim 19 wherein communication from the external system to thedevice is initiated by the external system communicating an inputcommunication signal comprising a voltage level substantially lower thanthe input low voltage threshold to the device.
 23. The method of claim22 wherein the input communication signal indicates that the externalsystem is available to communicate with the device.
 24. The method ofclaim 19 wherein communication from the device to the external system isinitiated by the device communicating an output communication signalcomprising an output current substantially greater than the input highthreshold to the external system.
 25. The method of claim 24 wherein theoutput communication signal indicates that the device is available tocommunicate with the external system.